Japanese Patent Laid-open Publication No. JP2001-130751 discloses a typical configuration of an automatic coating system including an electrostatic atomizer. Metered feeding of paint to the electrostatic atomizer is effected by a pump, which expels the paint from an external paint source toward the electrostatic atomizer, and a valve, which is housed in a cabinet as a separate member from the paint electrostatic atomizer.
Outline of the automatic coating system is explained with reference to the publication No. JP2001-130751. An electrostatic atomizer is attached to an arm of a robot located on an automatic coating line. The electrostatic atomizer communicates with an external compressed air source and paint tanks of paints of different colors. The valve cabinet houses a number of solenoid valves. Paint in an external paint tank is supplied under pressure by a pump toward the electrostatic atomizer, and metered feeding of paint is substantially controlled by a solenoid valve in the valve cabinet.
When a work is brought to a predetermined position of the automatic coating line, solenoid valves in the valve cabinet are controlled in motion, and a paint passage for a paint of a certain color to coat the work is opened to supply the paint under pressure through the paint passage. Then, the electrostatic atomizer is activated to atomize the paint while receiving supplemental supply of the paint.
Coating robots used in automatic coating lines for vehicles, for example, are commonly used to paint some desired colors. Therefore, every time after a coating robot finishes coating of one vehicle of a certain color, it needs procedures for changing the color to paint the next vehicle of a different color.
Japanese Patent Laid-open Publications No. JP-H08-229446 and No. JP-H11-262696 propose cartridge-type electrostatic atomizers. Publication No. JP-H08-229446 proposes to detachably attach a paint tank unit, having a metered paint feeding means inside, to the electrostatic atomizer. Publication No. JP-H11-262696 proposes to use feed units for respective colors, each having a valve inside, and to removably attach them to the electrostatic atomizer.
The cartridge-type atomizer proposed by the publication No. JP-H08-229446 is explained here. The paint tank unit containing a given quantity of paint is detachably attached to the atomizer, and after completion of a coating process, it is detached to wash the internal paint passage of the atomizer. This cartridge-type electrostatic atomizer has the advantage of shortening the paint passage to be washed upon a change of color, and therefore contributes to saving the quantity of paint washed away from the paint passage.
Problems of the atomizer taught by the publication No. JP-H08-229446 are pointed out here. In this known electrostatic atomizer, a combination of a fluid-driven piston and a cylinder is shown as a metered paint feeding means provided inside the paint tank unit. The paint contained in the cartridge-type paint tank unit is sprayed out of the atomizer by extruding it with the piston in the paint tank unit. However, since combinations of fluid-driven pistons and cylinders, in general, need high mechanical accuracy, here is the problem that the manufacturing cost of the cartridge paint tank unit increases.
If plastic materials are used to form the piston and the cylinder for reduction of the manufacturing cost of the cartridge-type paint tank unit, then the plastic piston and cylinder may absorb and expand with the pain and working fluids, and may change in size. Then, the piston and the cylinder, if made with strict accuracy, will cling to each other and do not work.
In case the piston and the cylinder is made of metals, the electrostatic capacity inevitably increases. Therefore, a relatively thick insulating layer must be provided to prevent leakage of static electricity from the cartridge-type pain tank unit. Here is the problem that the outer dimension of the unit increases.
A common problem involved in systems configured to supply pains from external pain tanks and systems configured to supply paints from cartridge-type paint tank units is that, because the rising characteristics upon starting atomization and the trailing characteristics upon stopping atomization are relatively dull, useless consumption of paints occurs upon starting and stopping atomization.
The electrostatic atomizer is halter during the period from completion of coating of a vehicle (work) to the start of coating of the next work. In this halt period, an amount of the plaint remaining under pressure in the electrostatic atomizer may leak through the paint outlet under the atmospheric presser.